Near field communication (NFC) very high bitrate (VHBR) phase shift key (PSK) modulation is typically used for NFC or smart card communication. PSK is a digital modulation scheme that conveys data by changing, or modulating, the phase of a reference signal. Typically a pattern of bits forms a symbol, which is then represented by a particular phase.
A convenient method to represent the phases used in PSK schemes is on a constellation diagram. Such diagrams are a representation of a signal modulated by a digital modulation scheme such as phase-shift keying. A typical constellation diagram shows the phase points in the complex plane with real and imaginary axes representing the in-phase and quadrature axes respectively. The amplitude of each point along the in-phase axis is used to modulate the signal.
In PSK, constellation points chosen are typically aligned with uniform angular spacing around a circle. This provides the maximum phase-separation between adjacent points and also gives the best immunity to corruption. Two common examples are “binary phase-shift keying” (BPSK) which uses two phases, and “quadrature phase-shift keying” (QPSK) which uses four phases, although any number of phases (xPSK) may be used.
As an example, an x Phase shift key modulation may be undertaken, where x is up to 16, using a small segment degree circle to implement the modulation. The segment circle is 60 degrees from −32 degrees to 28 degrees. Due to the very high PSK order typically used, the small circle segment, very high symbol rate and low pass channel characteristics, Automatic Gain Control (AGC) and Channel Estimation are highly required to have an ideal receiving performance.
A typical PSK modulation adopts 140 fixed symbol patterns for AGC and channel estimation, in which 48 symbols are periodic symbols for synchronization and AGC. Because of the very high PSK order—up to 16, and limited circle segment—60 deg, the constellation distances may be as small as 4 degrees. This leads to only a 2 degree noise margin. One issue that may arise from these conditions is misalignment of the memory of the receiver with the transmitted signal.
To save power consumption in smartcard applications, only one sample per symbol is allowed. In typical smartcards, a limiter function is needed to regulate the incoming power. The limiter function acts to destroy the amplitude information as such leading to a phase only receiver. On top of that, the system sometimes needs to operate in highly de-tuned cases.
Misalignment may also occur in other devices, such as for Near Field Communication (NFC) receivers. NFC receivers typically employ In-phase Quadrature (IQ) receivers. Misalignment may also occur for such systems and receivers due to the use of an analog mixer, which results in a delay between the in-phase or quadrature signals.
In a real application case, such as for a smartcard system or for NFC devices, all of these factors may easily cause the receiver to become out of phase with the transmitter, even during a preamble period. Accordingly, AGC and Channel Estimation after this point will be completely misaligned. The most common case of an out of phase receiver/transmitter arrangement is a 1 symbol misalignment between the receiver and transmitter. This is generally due to a misdetection of the first symbol as the low pass channel settles down.
A typical way to correct the misalignment in this type of communication system is to calculate the pattern of the received signal power. In particular, this information may be found in the initial preamble and then periodically as a reinserted preamble. As an example, in one communication system, the symbols may have values of 24, −24, 24, −24 or 28, −28, 28, −28 degrees. This pattern detection involves intensive computation with associated hardware cost and high power consumption and is also an error prone implementation due to the power threshold selection and memory address fix calculation mechanism. This disclosure is trying to utilize preamble pattern specialty and proposes a phase variance trend detection technique and module to detect and correct any detected misalignment.